Study setting and patient selection

This prospective cohort study was conducted at the department of oral and maxillofacial sciences of the study university in Rome. The period of recruitment was from November 2001 to January 2002. Patients were selected consecutively and treated from February 2002 to December 2003. After delivery of the definitive restorations, all patients were monitored annually and received supportive periodontal therapy. The patients were observed until July 2014. The STROBE (Strengthening the Reporting of Observational Studies in Epidemiology) guidelines for prospective cohort studies were followed. This clinical investigation was conducted in accordance with the ethical principles of the World Medical Association Declaration of Helsinki and was undertaken after informing the patients of the content, risks, and benefits of the study; written consent was obtained from each participant. The investigation was reviewed and approved by the local ethics committee.

The main inclusion criteria were that the patient be systemically healthy, totally edentate, and in need of an implant-prosthetic rehabilitation. A vertical bone height of at least 10 mm, adequate buccopalatal or buccolingual dimensions at the planned implant site, and a minimum of six implants planned in the upper arch or five implants planned in the lower arch were also required criteria. The patients were in a stable occlusal relationship with no parafunctional habits (clenching and/or bruxing) and the implant sites were free of infection and/or tooth remnants.

Exclusion criteria were alcohol or drug abuse and general health conditions that did not permit a surgical procedure. Local contraindications were, for example, tumours and ulcers. In addition, reason to believe that the treatment might have a negative effect on the patient’s psychological situation was also considered an exclusion criterion. A smoking habit was not considered a contraindication and was defined as smoking more than 10 cigarettes per day. The same operator (M.C.) performed the virtual surgical planning and surgical procedures. The treating clinician was an expert in implant dentistry and in immediate loading, but not in the use of multiple stereolithographic surgical templates.

Procedures (surgical and prosthetic phases)

The protocol employed in this clinical study consisted of an integrated treatment sequence that included the steps outlined below.

• (1)

  Creation of a radiopaque diagnostic template, a ‘scanno-guide’. This represented an exact replica of a temporary removable complete denture that was accepted by the patient and fulfilled the aesthetic and functional requirements.

• (2)

  Computed tomography (CT) scan of the patient’s arch. This was obtained with a spiral CT device (Asteion Multi; Toshiba Medical Systems, Rome, Italy). The scanno-guide was worn to integrate the anatomical data with the functional and aesthetic requirements. The CT examination was performed at least 1 month after the last tooth extraction.

• (3)

  Digital 3D CT-based surgical planning. The computer program employed in this study was SimPlant (Materialise Dental, Leuven, Belgium). Using this software, the implants were placed virtually, according to the bone anatomy and prosthetic designs. The potential receptor sites were located, and after evaluating the volume of available bone, the implants were placed with their corresponding abutment extensions. In the planning phase, disparallelism between abutments greater than 20° was avoided in order to reduce the technical difficulties in the laboratory phase. Each potential implant site was also evaluated for bone density using the Hounsfield scale (Hounsfield Units, HU) to assess its potential for immediate loading.

• (4)

  Development of a CAD design of the stereolithographic surgical template. This was performed by the clinician, in accordance with the patient’s prosthetic and anatomical needs.

• (5)

  Computer-aided manufacturing (CAM) of multiple stereolithographic surgical templates (SurgiGuide; Materialise Dental, Leuven, Belgium) to transfer the digital plan to the surgical environment. The surgical guides were employed in each patient to accommodate the three specified drills of increasing diameter used for osteotomy preparation. Therefore, three templates were created, characterized by 5-mm-high stainless steel tubes 0.2 mm wider than each drill. A fourth template was ordered with 5-mm-high stainless steel tubes 0.2 mm wider than the diameter of corresponding planned implant mounting devices.

• (6)

  Construction of a virtual plaster cast. Once the series of templates had been received, the fourth template was used to prepare the working cast, which was then used to create the temporary prosthesis. The plaster cast was characterized by the presence of the analogues of the implants included in the cast. The mucosa-supported template faithfully replicated the patient’s edentulous arch. SimPlant software allowed the thickness of the mucosa corresponding to each implant site to be known. The mounting devices were placed in the template’s steel tube, emerging from the tube with a distance corresponding to the thickness of the mucosa. The mounting device was locked with adhesive wax and the implant analogues were screwed into the mounting devices that were placed through the template. Plaster was poured into the template and the resulting internally hexed implant replica was placed into the plaster cast in a 3D position similar to that programmed virtually. To facilitate the prosthetic phase and to synchronize the position of the abutment within the analogue/implant, the flat side of the internal hex was rotated to the facial surface. Then, a plaster cast with the implants inserted was obtained before the computer-guided surgery: a so-called ‘virtual plaster cast’. Transfer of the intermaxillary relationships was obtained using the scan template (‘scanno-guide’). After mounting the plaster casts in the articulator, a new diagnostic wax-up was performed, the titanium abutments were milled, and the metal-reinforced full-arch provisional acrylic restorations manufactured ( Fig. 1 A–D) . The mucosa-supported guides did not require a flap reflection, but employed a flapless/transmucosal approach. Once the preoperative laboratory phase was completed, the patient was scheduled for surgery.

  

  Images from a male 49-year-old smoker affected by periodontal disease and requiring an implant-prosthetic rehabilitation. (A) Preoperative panoramic radiograph. All teeth were extracted and the patient was rehabilitated with upper and lower complete temporary dentures. After the extractions, the temporary dentures were relined and duplicated using barium sulphate. (B) A stereolithographic surgical template was used to manufacture the ‘virtual plaster cast’; the mounting devices and the analogues of implants fixed through the template steel tubes are shown. (C) A virtual plaster cast and the milled abutments. (D) The provisional restorations. (E) The insertion of 12 implants in the upper arch, and (F) 10 implants in the lower arch, and evaluation of primary stability using resonance frequency analysis (RFA). (G) Abutment milling and (H) delivery of the provisional restorations.

• (7)

  Computer-aided surgery. This was executed in accordance with the protocols of the implant system used. One hundred and eighty-eight cylindrical implants (Premium; Sweden and Martina, Due Carrare, Padova, Italy) with an internal hexagon (diameters ranging from 3.5 mm to 5.0 mm and lengths from 10.0 mm to 18.0 mm) were inserted in completely edentulous patients, using multiple mucosa-supported stereolithographic templates. The multiple mucosa-supported stereolithographic templates allowed the implant site preparation to be controlled, without depth control ( Fig. 1 E and F). Once the drilling sequence had been completed, the implants were placed into each osteotomy and rotated to the final depth, with the flat side of the hex positioned facially.

• (8)

  Immediate loading. The primary implant stability of the inserted implants was determined by evaluating the insertion torque using a manual calibrated torque gauge ratchet (Sweden and Martina, Due Carrare, Padova, Italy) and by resonance frequency analysis (RFA) (Osstell, Integration Diagnostics, Savadaled, Sweden). Implants characterized by an insertion torque of at least 30 N·cm and by an implant stability quotient (ISQ) value of ≥60 were immediately loaded ( Fig. 1 F). The abutments were positioned properly. Baseline postoperative peri-apical radiographs were taken for every patient immediately after abutment positioning to control the reciprocal fixture–abutment adaptation, which cannot be assessed directly. Using a flapless technique, the interposition of soft tissue may prevent the proper implant–abutment fit and lead to difficulty adapting the temporary prosthesis, with a subsequent error in the construction of the final restoration. Abutment parallelism was assessed and the abutments milled when necessary ( Fig. 1 G). During the adjustment, selective trimming of the temporary prosthesis was sometimes necessary, without severely affecting the rigidity of the whole bridge. The provisional restoration was then adapted to the titanium abutments with cold-cured acrylic resin ( Fig. 1 H). Care was taken to evaluate the arch midline, vertical dimension, and incisal show continuously. Determination of the occlusal plane was also important when the maxilla and mandible were reconstructed simultaneously. After completing and polishing the temporary prosthesis, an impression was taken at the abutment level for manufacturing of the definitive prosthesis. Further attention was given to avoiding the retention of impression material between the threads of the flapless inserted implants. The temporary restoration was seated and adjusted to remove any lateral, protrusive, and occlusal interferences. Centric and lateral contacts were assessed with articulating paper until light occlusal contacts, uniformly distributed on the entire prosthetic arch, were obtained. The prosthesis was cemented and remained in place for approximately 2 weeks – the time taken to delivery of the definitive metal–ceramic, cement-retained, full-arch prosthesis. A soft diet was suggested during the provisional prosthesis stage.

Assessment of marginal bone

The marginal bone level was recorded by taking standardized peri-apical radiographs, obtained with the use of the long-cone parallel technique and a Rinn XCP film holding system (Rinn XCP; Dentsply Rinn, Elgin, IL, USA). Care was taken to align the X-ray film in the film holder parallel to the long axis of the implants. Digital radiographs were stored using a digital intraoral imaging system (DenOptix QST Digital X-ray Phosphor Plate System; Gendex, Hatfield, PA, USA). The stored images were displayed on a monitor and direct measurements were performed using the software VixWin PRO (Gendex, Hatfield, PA, USA). Linear measurements from the fixture–abutment junction to crestal bone level were obtained mesially and distally using the software programme to analyze each image. Measurements taken from the radiographs obtained at the time of implant placement (T0) were used as baseline values. Baseline measurements (T0) were used to determine the amount of crestal bone remodelling at the 10-year follow-up (T1). To make calibrated measurements, an object of known size was used, e.g. the known length of the implant. Two researchers evaluated the radiographs independently; these researchers were not involved in the clinical part of the investigation. Inter-examiner and intra-examiner reliability was determined. Kappa statistics were used to compute intra- and inter-examiner reliability. The intra-examiner kappa coefficients were 0.84 and 0.88. The inter-examiner kappa coefficient was 0.78.

Implant survival criteria

Survival of the inserted implants was assessed at the 10-year follow-up. The assessment of implant survival and failure was based on the criteria suggested by Buser et al. and Cochran et al.

Prosthesis success and survival

The prosthesis survival and success rates were evaluated at the 10-year follow-up. Prosthesis success was evaluated using modified evaluation criteria suggested by the California Dental Association and described by Pozzi et al. A ‘surviving prosthesis’ was considered a prosthetic reconstruction that was stable and in good function.

Study variables

The predictor variables, i.e. clinical exposures or clinical risk factors considered to influence marginal bone loss, were grouped into the following categories: (1) biologically relevant variables: (a) sex (male or female), (b) smoking habit (smoker or non-smoker); a smoker was defined as a patient who smoked more than 10 cigarettes per day. (2) Anatomical variables: (a) arch (maxilla or mandible), (b) implant location (anterior (incisor and canine area) or posterior (premolar and molar area)). (3) Implant-related variables: (a) implant length (10.0, 11.5, 13.0, 15.0, or 18.0 mm), (b) implant diameter (3.5, 4.2, or 5.0 mm).

Minimization of potential sources of bias

In order to reduce potential sources of bias, the same operator performed the virtual surgical planning and the surgical and prosthetic treatments (M.C.). The same operator, expert in implantology and prosthodontics, completed all the follow-up assessments.

Statistical analysis

Statistical analyses were performed using IBM SPSS Statistics version 20.0 software (IBM Corp., Armonk, NY, USA). Descriptive statistics, including mean values and standard deviations, were used for the presentation of demographic study variables. The statistical analysis was performed at the implant level. Two-way analysis of variance (ANOVA) with Bonferroni adjusted post hoc test was used to evaluate the influence of different variables on the crestal bone levels. If any of the interaction terms was significant in the two-way ANOVA, then one-way ANOVA with Tukey’s range test was used to analyze the significant variables with more than two clusters. The unpaired Student t -test was used to analyze the significant variables with only two clusters. For all the tests performed, the significance level was set at P < 0.05.